I get the subjective impression that some LR operations are now slower, such as changing the parameters on a graduated filter........ Could the job of rendering more pixels to a larger monitor slow things down to a noticible extent?
Yes, I saw the same myself when I bought a 27" monitor :-(
Ironically the reverse also applies, so use a small screen and performance increases. Which, I assume, is why LR still works usefully well on netbooks*.
It has probably also pained the Adobe techs that so often recently people complaining about LR4's terrible performance have given lots of computer details, but rarely have detailed their screen resolutions.
*insert rant about dropping XP support to add useless video features.